U.S. Pat. No. 3,200,149 discloses the production of .alpha.-6-deoxytetracycline derivatives by a process which involves hydrogenation of certain 6-deoxy-6-demethyl-6-methylenetetracyclines in the presence of a catalytic amount of a noble metal catalyst such as rhodium or palladium. That process coproduces .beta.-6-deoxytetracyclines, as well as .alpha.-6-deoxytetracyclines. One of the major objectives of the present invention was to obtain a substitute catalyst which would produce a higher .alpha.-isomer to .beta.-isomer ratio.
It has recently been discovered that the diaceto(triphenylphosphine)rhodium(II) complex of the formula EQU Rh(OCOCH.sub.3).sub.2 (P[C.sub.6 H.sub.5 ].sub.3)
as described by Stephenson et al, J. Chem. Soc. 3632-40 (1965) is stereospecific in the hydrogenation of 6-deoxy-6-demethyl-6 -methylene-5-oxytetracycline to the extent that the ratio of .alpha.- to .beta.-isomers of 6-deoxy-5-oxytetracycline is greater than 9:1. The catalyst of the present invention which is of unknown structure is stereospecific to about the same extent but is not of the same structure as the aforesaid Stephenson et al catalyst. This can be demonstrated by observing, for example, that whereas the carbon content of the catalyst of the present invention is about 67.77%, the carbon content of Stephenson et al is but 54.6% and by the absence of the OCOCH.sub.3 absorption in the NMR and IR spectra.
There are a number of other hydrogenation catalysts known in the prior art which are of the general formula EQU RhH.sub.m Cl.sub.n (CO).sub.o (Ph.sub.3 P).sub.p
Wherein m, n, o and p range from 0 to 3. A variety of such catalysts have been prepared and either on the basis of color, melting point, IR spectrum or carbon and hydrogen and chlorine percentages, it has been found that these are not the catalysts of the instant invention.